Excess global warming that is currently taking place is thought to be largely caused by human activity increasing the “greenhouse effect”. Since the industrial revolution anthropogenic emissions have increased the amount of greenhouse gasses present in the atmosphere. In particular, combustion of fossil fuels has led to an increase in the atmospheric concentration of the greenhouse gas CO2.
In order to mitigate the effects of global warming caused by atmospheric CO2, attempts have been made to capture and sequester carbon. CO2 can be captured at point sources, such as power or cement plants, to prevent it being released into the atmosphere or it can be removed from the atmosphere at remote sites with technologies that remove CO2 directly from the air.
Once captured, the CO, can be stored in a number of ways, for example in deep geological formations, in deep ocean masses, in the form of mineral carbonates or in the form of bio-char. In the case of deep ocean storage, there is a risk of re-emission and of greatly increasing the problem of ocean acidification, a problem that also stems from the excess of carbon dioxide already in the atmosphere and oceans. Geological formations are currently considered the most promising sequestration sites [1]. However, the use of limited geological sites requires transport of the CO2 in pipelines either as a gas or as a supercritical liquid. CO, storage in geological formations is therefore associated with further energy consumption to transport the CO2 and to inject it into underground geological formations. Leakage of the stored carbon is also a major concern with both ocean and geological carbon sequestration.
Mineral sequestration traps carbon in the form of solid carbonate metal salts. One way to sequester carbon as carbonates is to use algae, in particular coccolithophorid algae, which are marine algae that form CaCO3 coccoliths. These algae take up CO2 from the atmosphere to form coccoliths, thus removing CO2 from the atmosphere and storing it in mineral form. Natural ocean-based coccolithophorid algal blooms are a well documented method of carbon fixation in coccoliths as is evidenced by many limestone deposits worldwide. However, ocean blooms of coccolithophorid algae are unpredictable, and the algae cannot be harvested to ensure that the sequestered carbon is stored long-term to prevent its re-release back into the environment via remineralisation of the algae after they complete their periodic and unpredictable, growth cycle. Natural blooms do not suffice to compensate for the increased atmospheric CO2 [2] and cannot be controlled.
There therefore remains a pressing need to develop an energy-neutral, robust and long-term method to sequester atmospheric CO2.